1. Field of the Invention
The present invention relates generally to methods for enhancing the recovery of oil.
2. Background Art
Hydrocarbons obtained from subterranean (e.g., sedimentary) formations are often used as energy resources, as feedstocks, and as consumer products. There are three stages of oil recovery from a formation. When oil wells are first drilled, the oil may flow up freely under its own pressure. At such primary recovery stage, oil and gas are produced using the natural pressure of the reservoir as the driving force to push the material to the surface.
At some point, the in situ pressure will decrease and the spontaneous production of hydrocarbons will cease, leading to the secondary recovery stage. When this happens, wells may need to be “stimulated.” Methods for well stimulation may include gas/fluid injection and water flooding, to produce residual oil and gas remaining after the primary recovery phase. U.S. Pat. No. 6,966,374 issued to Vinegar et al. discloses a method of using gas to increase the mobility of hydrocarbons in a formation.
Carbon dioxide is commonly used in gas injection. Pressurized CO2 has physical properties that enable it to extract hard-to-get oil trapped in an oil field's porous rock after the first stage of crude oil production. In this process, compressors inject CO2 into the oil reservoir, where the remaining oil and CO2 may chemically react to produce a modified crude oil that is now able to move more easily through the porous rock and toward oil production wells. In addition, water or steam injection is also commonly used to increase the oil pressure and/or improve oil viscosity to enhance production. Other methods of enhancing oil recovery includes heating the oil and making it less viscous, allowing it to flow out of the matrix and down into the fractures.
When oil production ceases after the secondary production, the wells may be further stimulated to afford tertiary recovery of the remaining oils. Tertiary recovery may involve injecting gases (such as carbon dioxide), or heat (steam or hot water) to stimulate oil and gas flow to produce remaining fluids that were not extracted during primary or secondary recovery phases.
During the third stage of hydrocarbon production, sophisticated techniques that alter the original properties of the oil may be used. Three major types of enhanced oil recovery (EOR) operations are in common use: (1) chemical flooding (alkaline flooding or micellar-polymer flooding), (2) miscible displacement (carbon dioxide (CO2) injection or hydrocarbon injection), and (3) thermal recovery (steam flood or in situ combustion). The selection of any of these methods depends on reservoir temperature, pressure, depth, net pay, permeability, residual oil and water saturations, porosity and fluid properties such as oil API gravity and viscosity.
To enhance oil recovery, chemical and/or physical properties of hydrocarbons within a subterranean formation may need to be changed to allow hydrocarbon material to be more easily removed from the subterranean formation. The chemical and physical changes may be induced by in situ reactions that produce removable fluids, composition changes, solubility changes, phase changes, and/or viscosity changes of the hydrocarbons within the formation.
For example, in situ thermal combustion of hydrocarbons (often used for recovery of heavy oils and tars) for enhanced oil recovery has been known in the art. Such processes may use external movable heating elements to heat a formation zone in the wellbore to increase the mobility of hydrocarbons. U.S. Pat. No. 6,902,004 issued to de Rouffignac et al. discloses the use of movable heater elements to raise the temperatures in portions of the formation to pyrolysis temperature to gain access to desired hydrocarbon blends in situ. U.S. Pat. No. 6,991,033 issued to Wellington, et al. describes the use of an in situ thermal process in which both the heat applied and the pressure are carefully controlled.
Some in situ thermal processes may use catalysts in “flameless combustors” to generate heat in the wellbore. U.S. Pat. No. 5,899,269 issued to Wellington et al. describes the use of a flameless combustor which contains a chamber coated with a catalytic surface of palladium or platinum metal.
In situ combustion or heating of heavy oils and tars may also be used to provide a means of partially breaking down very large hydrocarbon sources into smaller manageable ones and/or to reduce viscosities and increase flow so that desirable hydrocarbon blends can be recovered at the well bore. In this approach, it is important that ignition and combustion temperatures are not so high that the amount of recoverable hydrocarbon is compromised. U.S. Pat. No. 6,918,442 issued to Wellington et al. describes an in situ thermal process in which a mixture of hydrogen, hydrocarbons and other fluids may be produced in a formation.
The conventional in situ combustive processes described above require relatively high temperatures to initiate the combustion reactions. This means external energy from the surface must be applied and costs of EOR processes are increased. It is, therefore, desirable to have methods that do not require external energy inputs from the surface to initiate or maintain the in situ combustion for EOR.